A man entered the shop in Montagu Street at about 9am while the cashier was counting money.
The man asked for cigarettes and, as the cashier turned away to get them, he allegedly took a bundle of notes off the counter and left the store.
Officers have released CCTV images of a man they believe may have information about the incident, which happened on Thursday, October 13.
Witnesses or anyone who recognises the man pictured are asked to call Northamptonshire Police on 101 or they can call Crimestoppers anonymously on 0800 555111.